The impact of heatwaves in Patagonia

Wednesday, 10 August 2016 02:00
Climate Change Series

By Martín Jacques-Coper

Translated by Katy Harris

Patagonia has been populated for thousands of years by a wide range of cultures that made it their home. For Europeans and foreigners, this land was for a long time considered a mysterious and puzzling place of vast dimensions. Undoubtedly, the fascination that they had for this region was connected to its marvelous diversity of landscapes: extensive deserts and pampas as well as impenetrable moist, lush forests alongside impressive glaciers. However, this geographical richness has traditionally been linked to one dominant climatic characteristic: the cold.

Instrumental observation: the key to climate research

It has been a long time since the first steps of scientific, and in particular meteorological, exploration were taken in Patagonia. This discipline focuses on describing the state of atmospheric weather in a predetermined moment. Toward the start of the 20th century, development of this science in Patagonia was made possible by the installation of the first instruments for observing atmospheric variables, such as temperature, precipitation, wind intensity, and pressure.

Good quality instrumental data taken over a long time span is fundamental to climate study. For the climate of a particular location we interpret the meteorological characteristics averaged over a long period of time. Nevertheless, up to the present day, the number of meteorological stations in Patagonia is relatively low and the observational records are slim. As a result, it has been necessary to overcome a variety of difficulties in order to achieve a greater understanding of the region’s climate phenomena.

With regard to paleoclimate, in other words, past climates –taking into consideration various centuries, and even millennia, before instrumental measurements began–, the lack of climate data can be partially resolved by using indirect climate indicators, called proxies. For example, dendrochronology, which is dedicated to the study of tree rings, and the analysis of ice cores, can give us clues about climate variation in the past. As for more recent times, since we have been able to rely on meteorological observations in several parts of the world (around the mid-19th century), we can combine this information with climate models to reconstruct the climate of the recent past. We call this data reanalysis.

Patagonia in a warming world

Global warming is an increasing trend in the Earth’s temperature, an observable trend for more than a century. But due to the aforementioned problems with instrumental data, and due to its climate diversity, it is relatively harder to determine the exact rate of warming for the different areas in Patagonia.

Although global warming projections for the 21st century show that it will have a relatively moderate increase compared to other regions in the world, nevertheless the increase in temperature is currently having a considerable natural and socioecological impact on Patagonia. Desertification is a problem already detected to the east of the Andes, a problem that could have serious implications for the habitats of several species and the future development of the human population.

The Northern and Southern Patagonian Ice Fields and their glaciers are particularly sensitive to temperature changes. Their ice masses have shown a tendency to decrease in the last decades. That said, we must use science to better understand the relationship between glaciers and climate. To that end, first we must understand how Patagonia’s climate has varied previously and is varying currently in different time scales.

In this article we will concentrate on temperature development in this region during the 20th century. In this period, such changes were caused by a combination of natural factors and disturbances in the climatic system caused by human activity. Distinguishing between the two causes is a difficult task that is still incomplete; there is currently a big push in climate research dedicated to developing this aspect.

In the south it doesn’t only get cold

One fundamental climate variable in Patagonia is wind. The particularly strong wind that blows from the South Pacific toward the continent in medium latitudes is called a westerly. This is, for the most part, responsible for carrying humidity toward western Patagonia. When it meets the southern Andes, it generates considerable orographic precipitation, that is to say, caused in part by the topography. This strong wind is also responsible for the desert-like nature of the pampas in eastern Patagonia.

In summer, westerly winds air the continent, bringing relatively cold air from the Pacific to a comparatively warmer continent, due to the concentrated sunshine in this season. Thus, if the trajectory or typical intensity of these ocean to continent winds is disturbed, temperature changes will be noted on the continent. Consequently, despite the general association with cold, Patagonia’s climate also comprises relatively warm events. For this article, we will concentrate on summer, the warmest season of the year.

Despite being commonly known for its cold temperatures, Patagonia’s climate also includes relatively warm periods.

Different time scales

The variability of climate, and particularly temperature, happens in several time scales (daily, seasonal, annual, decadal). For Patagonia, let us do the mental exercise of breaking down the temporal evolution of summer temperatures in the 20th century into these time scales, in such a way that, if we add together the components of these different scales, we rebuild the original variability.

So, in the first instance, we identify global warming alongside larger time scales. Then, beyond this sustained increase in temperature, we realize that some decades are warmer than others. In Patagonia, we see that the periods between the 1920s and part of the 1940s, and between the 1980s and the mid-1990s, were warmer than the decades in between. This is the decadal variability, covering dozens of years.

If we focus on an even shorter time scale, we can assume that, from one year to another (in our case: from one summer to another), there will also be considerable temperature variations in Patagonia. And this, effectively, is how it works: in this region, if one summer is relatively hot, the following one tends to be a little less so, or even ends up being comparatively colder. In this way, after 3 or 4 summers, there will once again be a relatively hot summer. So, for example, we see that the summer of 1960 was hot, 1961 was relatively colder, 1963 was hot again, 1965 was quite cold and 1967 was, once again, hot. Even though this seems to indicate an assured regularity, the real variability tends to be somewhat more irregular. It is important to emphasize that in this analysis we have discounted the direct effect of a temperature increase associated with global warming. In principle, this type of temperature variability in Patagonia occurs independently of such phenomenon. However, it is still necessary to explore in greater depth in case there is a connection between both aspects.

Hot summers in Patagonia? Well, yes, although let us remember that this term is relative to the long-term climate conditions that are particular to this region. As we might expect, temperature changes in Patagonia are partly due to a disturbance in the westerly winds. During hot summers, we observe that high atmospheric pressure –also known as anticyclone–, defined according to the average historic conditions of the regional climate, positions itself above Patagonia in a persistent manner. The anticyclone deflects the westerly winds southward. As a result, there is less ventilation on the continent during a hot summer, which results in a net warming of Patagonia.

The anomalous wind current resulting from the westerlies, deflected southward, crosses the Drake Passage and is then directed northward. This passes via the South Atlantic and then arrives in southern Brazil, before finally heading east and reaching the subtropical Atlantic. This trajectory unfolds, therefore, in an S shape (first eastward, then northward, then eastward), and is partially linked to frosts in the subtropics, but this is another story. The high pressure center over Patagonia not only causes the westerlies to deflect, but also favors stable atmospheric conditions and clear skies. This leads to incidental solar radiation over the surface of the continent, which warms it further.

The connection between Patagonia, the South Pacific and Oceania

The fact that high pressure over Patagonia, persistent during hot summers, is not generally an isolated atmospheric anomaly is especially relevant. The aforementioned anticyclone is part of a spatial pattern of high and low pressure centers that alternate over the South Pacific, from Oceania towards South America. In climatology this is called a wave train. In accordance with this result, when an anticyclone induces a hot summer in Patagonia, a cyclone (that is to say, low pressure) is usually observed over New Zealand and the Tasman Sea, which separates the country from Australia. In Oceania, the atmospheric circulation associated with this cyclone tends to cause dry conditions in southeastern Australia.

We can therefore conclude that summers that are hot in Patagonia tend to be dry ones in southeastern Australia. This is not an infallible law, but a general rule throughout the 20th century. We call this climate link between two remote locations teleconnection. This discovery has some scientific implications. For example, if we could prove that this connection also happened further into the past, we would at the same time be able to reconstruct the climate of both distant regions.

Enough talk about climate, where does that leave meteorology? Up to now we have described the average conditions: what happens between decades or during a whole summer, which consists of around 90 days. However, in our daily routine, we are normally faced with the fact that meteorological weather changes from one day to the next. Let us concede that this is not valid in any old part of the world, but it is in Patagonia, which is what interests us on this occasion. What happens with the temperature variability during a summer, let alone decades and whole seasons? We call this intraseasonal scale. What would be, over a time scale of a few days, a similar situation to the hot summers, that happens over an interannual time scale? Well, a considerable increase in temperature, relative to average summer conditions, which takes place over a few days. We call this a heatwave. And, although we may find this surprising, this type of event does happen in Patagonia.

The characteristics of a heatwave in Patagonia depend on the way that we define it in relation to the criteria of persistence (duration) and intensity (level of temperature increase). In this case, we use reanalysis data to investigate the occurrence of these events from the end of the 19th century up to the present day. Our results indicate that, during a typical event, the temperature in Patagonia is higher than normal for a period of approximately two weeks and it reaches a maximum of more than 4°C in relation to the usual values. Furthermore, we observe that one or two heat waves happen every summer and that during the 20th century there was no clear tendency toward a greater frequency of these. However, this does not necessarily imply that climate change does not play a role in heatwaves developing. This is still a subject in need of more study in climatology.

In general, a heatwave does not occur instantaneously over Patagonia; these events tend to happen gradually. An increase in temperature begins in the west of the region. Then, the relatively hot zone expands and advances to the east. In a similar manner to what was described previously, the hot conditions in this case are also connected to high pressure in the region. This anticyclone determines the atmospheric circulation over Patagonia, that is to say, the winds. What’s more: the wave train over the South Pacific also appears on this scale! And this whole system advances over the Pacific from west to east, on a time scale of approximately ten days. What is fascinating, in this case, is that the teleconnection with Patagonia extends to Oceania and can be observed even further afield, including the tropical west Pacific. In other words, we conclude that certain meteorological phenomena that occur in this remote region can, after a few days, lead to the development of meteorological events in Patagonia, such as heatwaves. Such a result indicates that monitoring conditions in the tropical Pacific could help us to anticipate these events happening.

In summary, when there is a heatwave in Patagonia, we observe abnormally high regional temperature and pressure values. In a larger spatial context, we see the evolution of a wave train in the atmosphere, comprised of cyclones and anticyclones that alternate between Oceania and South America.

Heat in Patagonia: what are the consequences? We have briefly described some aspects related to heat conditions in Patagonia. Why the scientific interest in this subject? There are several reasons. As mentioned at the start of this article, climate change could have significant natural and socioecological consequences in Patagonia. In that regard, the measures taken for long-term land management and water resource planning can be done based at least in part on the results obtained from climatology. In the short term, heatwaves can create conditions that lead to forest fires, with devastating consequences for the region. We hope that future, more detailed research based on further data will allow us to further improve our ability to predict climate change, heatwaves and their impacts.

The author, Martín Jacques-Coper, a geophysicist, studied at the University of Chile and holds a doctorate in climate sciences from Bern University in Switzerland. His research interests include climate variability and dynamics, with a special focus on South America. This article was supported by an EcoPatagonia reporting grant from Patagon Journal in partnership with the Earth Journalism Network. More info at www.ecopatagonia.org